We propose to develop an electrotactile vestibular substitution system (ETVS) for persons with loss of balance due to vestibular loss. The artificial sensor (an accelerometer) as well as the tactile array of tongue stimulators will be built into a dental orthodontic retainer. This phase 2 proposal will build on successful Phase I results obtained with an early prototype in which the accelerometer was built into a hard hat and the information was led into the mouth via a ribbon cable, Subjects reported return of balance during the time of use of the vestibular substitution prototype; ETVS via the tongue provided both head and body stability. The three primary objectives are to: A. Develop a mobile tactile display system. For this project we will need a compact, wearable version of the system developed in Phase-l. This new prototype will be comprised of 2 subcomponents--an oral display unit containing the sensing and stimulation electronics that is tethered by a thin cable to the base unit that contains an embedded controller with programmable I/O capabilities for both data acquisition and operation code upgrades. B. Optimize the human-machine interface. We will perform a series of psychophysical experiments to (i) refine our understanding of tongue inhomogeneity with respect to sensitivity and dynamic range of electrotactile stimulation, and (ii) how to encode the specific head-body orientation cues that maximizes information transfer while minimizing sensory adaptation and discomfort. C. Perform functional trials. We will perform controlled tests at 3 intervals: both pre- and post-training and post-experience, with a group of BVD subjects. The effect of ETVS under various experimental conditions (e.g. sit-to-stand, quiet standing, and walking on a treadmill, each at normal, low, and no-vision) will be quantified using refinements of the posturographic and spectrographic techniques that we developed in Phase-1 for analysis of our experimental data. Data has been collected from potential users which indicate that there is a market for this device, and that will provide the practical experience to develop a totally in-mouth system hidden in a dental retainer. The ETVS may also have applications to: falls in the aged due to loss of balance; Menniere's disease; spatial disorientation in airplane pilots; basic research in postural control; a bedside diagnostic system; and a therapeutic system (our phase 1 studies have shown intriguing evidence for this. However, these applications will not be included in this proposal).